Buy a space heater to keep in your bedroom if you like it extra toasty. You can also buy thermal curtains to hang over the windows to keep the chill out. If there is a gap between the floor and your door, stuff some fabric or cushion between it too.

Try turning the heat down when you're not in the apartment during the day, or at night when you go to sleep. It's not worth keeping your apartment at the same temperature when you're not there, so this is a good way to cut back. Buying a humidifier can also help, as moist air tends to retain heat better, so your apartment will stay warmer.